It is generally recognized that nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O) are principle contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. NOx is the term generally used to represent nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), as well as mixtures containing these gases. NOx forms in the high temperature zones of combustion processes. The internal combustion engine, and coal or gas-fired or oil-fired furnaces, boilers and incinerators, all contribute to NOx emissions. Although the concentrations of NOx in the exhaust gases produced by combustion usually are low, the aggregate amount discharged in industrial and/or highly populated areas tends to cause problems. NOx is also produced during a variety of chemical processes such as the manufacture of nitric acid, the nitration of organic chemicals, the production of adipic acid, and the reprocessing of spent nuclear fuel rods.
In general, fuel-rich combustion mixtures produce exhaust gases with less NOx than do lean fuel-air mixtures, i.e. mixtures in which more air is provided than the stoichiometric amount required to completely combust the fuel. Lean fuel mixtures will produce an exhaust gas that contains gaseous oxygen.
The US Environmental Protection Agency is requiring greater levels of NOx abatement from mobile and stationary emission sources. For ‘light duty’ mobile sources such as light trucks and cars, NOx emissions will be required to not exceed 0.07 grams/mile, down from the current level of around 0.8 grams/mile. This represents a NOx abatement requirement of greater than 90% over current technology. Some of this abatement will come from advanced vehicle design and advances in combustion technology, but most of the reduction will come from advanced emission controls of which NOx reduction catalysts are the central technology. Similar reductions will be required of heavy diesel trucks in the near future, hence the need for new technologies having the capability of achieving very high reduction of NOx from lean burn engines, and at low operating temperatures as well (150° C.-250° C.).
Although the NOx gases may be thermodynamically unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method or catalyst has been described for inducing this decomposition at high enough rates over broad temperature ranges to make lean NOx reduction economically feasible. It has been discovered, however, that the addition of a reductant such as ammonia to the exhaust gas, under appropriate conditions, converts NOx to elemental nitrogen and steam.
The process of contacting an industrial flue gas with a catalyst in the presence of ammonia at a temperature usually in the range of about 200 degrees Celsius to about 600 degrees Celsius to reduce NOx in the flue gas is commonly known as the process for Selective Catalytic Reduction (SCR) of NOx. Any reference made herein to “Selective Catalytic Reduction,” or to “SCR,” is intended to refer to a process in which a mixture of NOx and NH3 are induced to react catalytically in the presence of oxygen at elevated temperatures.
For lean burn engine technology to be implemented, catalytic converters must be developed for lean burn engines. The catalyst employed in the converter must be active over a broad range of temperature (usually in the range of about 150-500 degrees Celsius, or broader is better), must have very high activity for the conversion of NOx to elemental nitrogen (N2) and water (H2O), must react with a broad range of NO and NO2 in the gas sent from the engine to the catalytic converter, must be sulfur tolerant, and should not produce N2O or only a few ppm at most. With these goals in mind, lean burn catalysts that remove NOx from exhaust streams (i.e. deNOx catalysts) are highly sought after and are the focus of considerable research worldwide.
A rather narrow window of satisfactory operating temperatures has characterized most catalysts for lean burn applications. Specifically, they only effectively convert NOx over narrow temperature ranges that do not always match the temperatures at which the NOx is emitted. Some of the better catalyst materials have included metal-substituted zeolite catalysts such as Cu-ZSM-5, Fe-ZSM-5, and related catalysts consisting of various zeolites with metal ions substituted into the zeolite structure. These materials are better in some ways than conventional platinum-based deNOx catalysts, but usually the best operating temperature ranges are too high (above 400 degrees Celcius) and too narrow (only about a hundred degrees Celcius in effective temperature width) for many practical applications. One significant advantage that such ‘base metal’ (non-precious metal) zeolite catalysts have over Pt or other precious metal-containing catalysts (e.g. Pt, Pd, Rh, Ir) is that precious metal containing catalysts are known to produce copious quantities of N2O in addition to N2 under lean burn conditions. N2O emissions are not yet regulated, but because N2O is a potent greenhouse gas, it is a very undesirable byproduct, and a technologically useful catalyst should produce little, if any N2O.
It has been recognized that the mechanism of SCR of NOx in the presence of ammonia requires an approximately 1:1 ratio of NO to NO2 in the feed stream to achieve the highest rates of NOx reduction. Ratios above or below 1 result in significantly slower rates of NOx reduction. As combustion processes generate NOx mixtures having very high NO/NO2 ratios, i.e. the engine emits mostly NO, a process for reduction of the NO must include a method for oxidizing some of the NO to NO2; preferably about half of the NO should be converted, resulting in a nearly 1:1 ratio.
Emissions control systems for mobile applications are likely to have an oxidation catalyst upstream from the NOx reduction catalyst to oxidize unburned hydrocarbons. This oxidation catalyst will convert some of the NOx to NO2, perhaps up to 20 to 30 percent at a temperature of about 150 degrees Celsius, but not the 50 percent required to achieve the fastest rates of NOx reduction. Because most NOx catalysts are not capable of oxidizing NOx to NO2 at low temperature, these catalysts cannot assist the hydrocarbon oxidation catalyst to generate the advantageous mixture of NO/NO2 and so these catalysts are largely ineffective at low temperatures, that being below 300 degrees Celsius where the feed contains mostly NO.
A strategy for improving the low temperature activity of SCR catalysts is to provide an additional non-precious metal containing catalyst that can oxidize NO to NO2 so that the highest rates of NOx reduction can be realized. This is a strategy employed with the present invention.
In addition, internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a large fraction of the total emitted hydrocarbons released during the first minutes of engine operation are due to the uncombusted hydrocarbons. Such release of hydrocarbons after cold engine start-up poses a special problem, as at that point the temperatures of the exhaust gas and the catalytic converter are generally not high enough for conversion of the gaseous pollutants by conventional catalysts. The catalysts in present catalytic converter systems are generally ineffective at ambient temperatures and must reach high temperatures, often in the range of 300 degrees Celcius to 400 degrees Celcius, before they become effective. During this time period, unburned hydrocarbons may adsorb onto the catalyst, causing a further diminution in activity. Indeed, under some circumstances, the adsorbed hydrocarbons may form carbonaceous deposits, requiring high temperatures to remove the deposit oxidatively. This can lead to irreversible damage of the catalyst. Therefore, catalysts that can avoid hydrocarbon deposition at low temperature, or more preferably, oxidize unburned hydrocarbons at the lower temperatures, are highly desired.
SCR processes offer the possibility that unspent ammonia reductant could be emitted to the environment. As ammonia is a regulated toxic substance, there are stringent emissions standards for ammonia. Therefore, another desired feature for a broad temperature range SCR process is one in which very little, if any ammonia is allowed to escape into the atmosphere, even under strenuous transient conditions where the process temperature is increasing rapidly because of load on the engine. In other words, the catalytic NOx reduction process should consume all of the ammonia, or the catalyst should consume any excess ammonia by oxidation. In the latter case, it would be highly advantageous if the oxidation of any excess ammonia did not result in the formation of more NOx, but that the oxidation process resulted in the net oxidation of ammonia to N2— a so-called selective catalytic oxidation process.
A number of zeolite-based catalysts for SCR of NOx with ammonia are described below. Many of these catalysts where their activity is given have been tested in the forms of powders or compacted powders. In these catalytic tests, the flows through the catalyst beds are given in terms of gas hourly space velocity (GHSV). The GHSV is the volume of exhaust passed in one hour divided by the volume of the catalyst bed, and is related to the residence time or reaction time that the gaseous species have to react on the catalyst before they leave the catalyst bed. It is generally desirable to minimize the catalyst volume to the extent possible, and a useful catalyst should have high activity at high GHSV. For combustion processes, the GHSV is typically in a range from about 20,000 h−1 to about 200,000 h−1. One difficulty in comparing the activity of one catalyst to another when relative flow rates are given in terms of GHSV arises when one tries to compare a compacted powder catalyst with a catalyst that is supported on a monolith. In a powder catalyst, the bed volume is measured in a straightforward manner. If the catalyst is supported on a monolith such as a commercial cordierite honeycomb support, then the catalyst volume is given as the volume of the honeycomb. The problem here is that the amount of catalyst supported on the honeycomb is very small; most of the volume of the honeycomb catalyst is void space and the volume of the honeycomb itself. This makes it very difficult to make a simple comparison of catalyst activity between a powder catalyst and a monolith-supported catalyst. A rule of thumb that is commonly used is to make a rough comparison in activity between a powder catalyst and a monolith catalyst is to multiply the GHSV of the powder catalyst by about 4, or conversely to divide the GHSV of the monolith catalyst test result by about 4. For example, if a powder catalyst is reported to have a certain activity at 30,000 h−1 GHSV, then it should be compared to a monolith catalyst at roughly 7,500 h−1 GHSV. Conversely, if a monolith catalyst has been reported to have a certain activity at 30,000 h−1 GHSV, then the powder catalyst should be compared at a GHSV of about 120,000 h−1.
The use of zeolite-based catalysts for the SCR of nitrogen oxides with ammonia is well established. U.S. Pat. No. 4,220,632 to D. R. Pence et al. entitled “Reduction of Nitrogen Oxides With Catalytic Acid Resistant Aluminosilicate Molecular Sieves and Ammonia,” incorporated by reference herein, discloses the catalytic reduction of noxious nitrogen oxides in a waste stream (stack gas from a fossil-fuel-fired power generation plant or other industrial plant off-gas stream) using ammonia as reductant in the presence of a zeolite catalyst in the hydrogen or sodium form having pore openings of about 3 to 10 Angstroms.
U.S. Pat. No. 4,778,665 to Krishnamurthy et al. entitled “Abatement of NOx in Exhaust Gases,” incorporated by reference herein, describes an SCR process for pretreating industrial exhaust gases contaminated with NOx in which the catalyst includes an intermediate pore zeolite with a silica to alumina ratio of at least 50 with a Constraint Index of 1 to 12. These zeolites are sometimes referred to as ZSM-5 type zeolites. The zeolite is preferably in the hydrogen form or has up to about 1 percent of a platinum group metal. According to the '665 patent, the hydrogen form of zeolite ZSM-5 (HZSM-5) catalyzes the SCR reaction at temperatures between about 400 degrees Celsius to about 500 degrees Celsius. At temperatures below about 400 degrees Celsius, HZSM-5 is significantly less efficient at removing nitrogen oxides from the gas stream. These catalysts were tested as compacted powder extrudates at space velocities below 10,000 h−1.
U.S. Pat. No. 5,520,895 to S. B. Sharma et al. entitled “Method for the Reduction of Nitrogen Oxides Using Iron Impregnated Zeolites,” which issued on May 28, 1996 and is hereby incorporated by reference, describes a process employing impregnated zeolites as catalysts for the SCR of NOx in exhaust gas. A catalyst used with this process includes an intermediate pore size zeolite powder that has been contacted with a water-soluble iron salt or salt precursor to produce an iron loading of at least 0.4 weight percent, and a binder such as titania, zirconia, or silica. The impregnated zeolite is calcined and hydrothermally treated at a temperature of about 400-850 degrees Celsius to produce a catalyst that is capable of greater than 80 percent conversion of the NOx to innocuous compounds when the catalyst has been aged using 100 percent steam at 700 degrees Celsius for 7 hours prior to sending the exhaust gas over the catalyst. These catalysts were tested as powders at space velocities of 12,000 h−1.
U.S. Pat. No. 6,514,470 to K. C. Ott et al. entitled “Catalysts for Lean Burn Engine Exhaust Abatement,” hereby incorporated by reference, describes the catalytic reduction of nitrogen oxides in an exhaust stream gas using a reductant material and an aluminum-silicate type catalyst having a minor amount of a metal. Nitrogen oxides were reduced in the exhaust stream gas by at least 60 percent at temperatures within the range of from about 200 degrees Celcius to about 400 degrees Celcius. The term “hydrocarbons” as it is used both in the '470 patent and in the present patent application, is meant to refer to hydrocarbons and also to partially oxidized products of hydrocarbons such as oxygenated hydrocarbons (alcohols, ketones, and the like). While hydrocarbons were used as the reductant in the examples disclosed in the '470 patent, it was mentioned that ammonia could also be used. However, no data using ammonia as reductant was presented.
In “A Superior Catalyst for Low-Temperature NO Reduction With NH3,” Chem. Communications, (2003), pp. 848-849, incorporated by reference herein, Gongshin Qi and Ralph T. Yang report an SCR process using a Mn—Ce mixed-oxide powder catalyst that yields nearly 100 percent NO conversion at 100-150 degrees Celsius at a space velocity of 42,000 h−1 (as a powder), and that SO2 and water have only slight effects on the activity. The catalyst activity was shown to be dependent on the relative amount of Mn in the catalyst, and also on the calcination temperature used to prepare the catalyst.
In “Low-Temperature SCR of NO With NH3 over USY-Supported Manganese Oxide-Based Catalysts,” Catalysis Letters, vol. 87, nos. 1-2, April 2003, pp. 67-71, incorporated by reference herein, Gongshin Qi, Ralph T. Yang, and Ramsay Chang report the SCR of NO with ammonia and excess oxygen using manganese oxide, manganese-cerium oxide, and manganese-iron oxide supported on USY (ultrastable (i.e. high Si/Al) Y zeolite). It was found that MnOx/USY had high activity and high selectivity to nitrogen at temperatures of from 80-180 degrees Celsius, and that the addition of iron oxide or cerium oxide increased NO conversion. A catalyst of 14% cerium and 6% manganese impregnated into ultrastable Y zeolite produced nearly 100 percent conversion of NO at 180 degrees Celsius at gas hourly space velocity (GHSV) of 30,000 h−1 as a powder catalyst. The only reported product was N2 (with no N2O) below 150 degrees Celsius.
There remains a need for catalysts that show better performance for SCR of NOx, especially catalysts that reduce NOx at temperatures less than about 200 degrees Celsius at high space velocities, particularly when supported on monolith honeycomb supports.
Therefore, an object of the present invention is to provide a catalyst for the selective catalytic reduction of NOx in the presence of ammonia that shows excellent conversion at temperatures below 200 degrees Celsius at space velocities greater than 30,000 h−1 when tested as a monolith-supported catalyst, or 120,000 h−1 when tested as a powder.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.